The present invention is directed toward a solid-state display device of matrix configuration, in which a switching element is provided on the display panel for each of the display elements.
At present, various types of solid-state display devices are in wide use in a broad range of electronic equipment. Such display devices generally utilize liquid crystal electro-optical effects, or other electro-optical effects including electrochromism or electroluminescense, and due to their advantages which include compactness, low power consumption and light weight, are especially suited to such applications as electronic timepieces, pocket calculators, and similar equipment. In future, it is anticipated that the applications of such display devices will be extended to, for example, miniature portable television receivers, computer peripheral equipment, etc. In the case of equipment such as television receivers, a high degree of display element density is required. In such cases, a matrix type of display arrangement is generally adopted, in order to minimize as far as possible the number of interconnecting leads between the display panel and the circuitry which drives the display elements, i.e. the peripheral circuitry. In such a display device, display elements are selected and driven by means of a plurality of row and column electrodes. However, it has been found that the level of cross-talk interference in a simple matrix display of liquid crystal type, for example, is too high to permit a usefully large size of matrix display to be utilized. Because of this, a method was proposed whereby an individual switching element is provided for each of the display elements. This method, which enables cross-talk to be virtually eliminated, is referred to as the "built-in switching element" method in the present disclosure, and was first proposed by B. J. Leichner et al (Proc. IEEE, Vol. 59, Nov. 1971, P. 1566 to 1599).
Theoretically, such a display device should provide a performance which is comparable to that which can be obtained with a cathode ray tube type of display, while also providing the advantages of low power dissipation and capability of operating at low levels of supply voltage. However, it has been found that, in practice, such performance has not been obtained with prior art display devices of the built-in switching element type, due to various disadvantages. One of these disadvantages is that the display elements have a finite resistance, and as a result of this, and also the leakage resistance of the switching elements coupled to the display elements, the contents of the display elements gradually deteriorate, i.e. the charge which is established in the display element when display data is written in gradually discharges. It is therefore necessary to periodically and repetitively rewrite the display data into the display elements, on a continuous basis. If the display data is not available continuously from an external source, then this means that it is necessary to provide some type of memory means to store the display data, i.e. a memory whose capacity is at least as great as the number of display elements. In the case of a high-density type of display, this requirement is a very serious disadvantage. Another disadvantage which results from the need to repetitively write in the display data to the display elements is that since deterioration of the display contents occurs at quite a rapid rate, it is necessary to rewrite the data fairly rapidly. Since the display data is generally transferred from an external source or from an internal memory in serial form, to be written in, it is necessary to provide some means of serial to parallel conversion to apply this data to the display matrix, and so this serial to parallel conversion circuitry must contain elements which operate at a high switching rate. This means that a substantial amount of power will be dissipated in such circuitry, and that the area required for the circuit elements will be substantial. In addition, since it is difficult to form thin-film semiconductor devices which operate at high switching speeds, it is not feasible to provide such serial to parallel conversion circuitry upon the display panel itself, in the same way as the switching elements of the display elements are formed.
Another disadvantage of such prior art display devices is that it is only possible to write in data to be displayed, and not to readout the contents of the display.
The various disadvantages of prior art display devices of matrix type having built-in switching elements are eliminated by a display device according to the present invention. Regeneration of the contents of the display elements is performed periodically and automatically on the basis of the contents themselves, and not on the basis of externally stored data. Thus, it is not necessary to use a memory circuit, or to provide serial to parallel conversion circuit means operating at high switching speeds. Thus, the overall configuration of a display device according to the present invention can be simplified, by comparison with prior art devices, and the size and power consumption can be reduced. In addition, data can be read out from the display elements of a display device of the present invention, as well as being written in. Thus, the display elements can be used as data storage elements. Moreover, it is possible to write in data to the display device by application of a light pen or other suitable light source to the display panel, in a similar manner to that of a cathode ray tube display device having a light pen data input capability.